With regard to a drum type washing machine, since a rotary drum is disposed in a water tub such that its rotational axis is horizontally oriented or slanted with respect to the horizontal direction with its front portion raised, laundry and water in the rotary drum tend to gather in the lower portion of the rotary drum when the rotary drum is rotated after loading laundry therein, the condition of which tends to cause excessive vibration of the rotary drum. Particularly, just as a water-extracting process is performed after the washing and rinsing processes, the laundry in the rotary drum contains water therein and may be unbalanced toward one portion of the rotary drum due to the rotation of the rotary drum depending on the types, materials and shapes of the laundry. If the laundry is placed in such an off-balanced state during the water-extracting process in which the rotary drum is rotated at a high rpm, the water tub accommodating the rotary drum therein would vibrate considerably, thereby causing abnormal vibration or noise. In case abnormal vibration occurs, a control process to remove the unbalanced distribution of the laundry in the rotary drum is performed by stopping the rotation of the rotary drum temporarily and then resuming the rotation thereof or by lowering the rpm of the rotary drum.
In order to perform a control operation in response to abnormal vibration, various methods have been proposed for detecting such abnormal vibration promptly. For example, Japanese Patent Laid-open Application No. H6-170080 (Reference 1; see pages 2 to 3, FIG. 1) discloses a method for detecting abnormal vibration of a washing machine. In this method, a warning of excessive vibration is outputted by detecting excessive vibration based on an output current of an inverter circuit, wherein the inverter circuit controls an induction motor which rotates the rotary drum.
Moreover, Japanese Patent Laid-open Application No. H61-098286 (Reference 2; see pages 1 to 3, FIG. 1) also discloses a method for operating a washing machine. In this method, the transition from the washing cycle to the water-extracting cycle is controlled based on a vibration detection output transmitted from a vibration detecting sensor for detecting the vibration of a water tub. Here, the water-extracting operation is stopped immediately once an abnormal vibration is detected during the water-extracting cycle.
However, the method of Reference 1, which detects abnormal vibration indirectly from variations in output currents of the inverter circuit, is based on the assumption that laundry's imbalanced state is reflected by an effective current of an induction motor and that the imbalanced state leads to abnormal vibration. However, the variations in the effective current of the induction motor can be caused not only by an unbalanced distribution of laundry in the rotary drum but also by various mechanical factors, e.g., a bearing of the induction motor or the like. Further, since a set value for current in detecting an excessive vibration is determined based on the variations of the effective current, excessive vibration warnings may be given unnecessarily, thereby stopping the rotary drum too frequently.
Further, in case of the method disclosed in Reference 2 in which a vibration of the water tub is detected by a vibration detecting sensor, during the balancing operation wherein the rotary drum is rotated at a low rpm, an abnormal vibration may not yet be detected due to small amplitudes of vibration, even if there is an unbalanced distribution of laundry in the rotary drum due to laundry's clinging to an inner surface of the rotary drum. Since the amplitudes of vibration are too small to be detected until the rotational speed of the rotary drum reaches a high rpm, it tends to be difficult to detect abnormal vibration until the rotary drum is rotated at a high rpm. Therefore, there is a greater risk that the laundry or the washing machine can be damaged by abnormal vibration as the rotary drum is brought to a stop, and it will take too much longer to decelerate the rotary drum spinning at a high rpm.